1. Field of the Invention
This invention relates to sound reproduction systems with electrodynamic loudspeakers and closed housings. More particularly, the invention relates to a sound reproduction system for improved bass reproduction.
2. Prior Art
Conventional loudspeaker systems have an inferior bass reproduction if the housings are small. In small housings air compression forces will build up and hinder the movement of the radiating loudspeaker's membrane. These forces evolve from volume changes in the air inside the housing which are caused by the movement of the loudspeaker's membrane. The membrane compresses or decompresses the air and the resulting forces hinder the movement of the membrane. Being elastic forces they also increase the resonance frequency of the system. To achieve a satisfying bass reproduction large, impractical housings are used, or different kinds of resonant boxes are employed. Often the driving signals are corrected in their frequency characteristic, or the loudspeakers are controlled by servo systems. All these solutions cause distortions or are impractical to use, or show a poor pulse response.
Another known method (Tiefenbrun, U.S. Pat. No. 4,008,374) uses a second loudspeaker incorporated into the housing to simulate a larger volume. However this method just transfers the problems from the outer to the inner loudspeaker. To achieve satisfying results large housings must be used once again. Additionally, problems arise from distortions caused by phase differences between the movements of the membranes.
Price Shelton's invention (Goodman, appl. GB.821 5906) follows Tiefenbrun's principle of using an inner transducer to simulate a larger inner volume. In addition Shelton places a pressure sensor into the inner chamber of the housing to measure pressure changes. The signal produced by the sensor is amplified by an operational amplifier and drives the inner transducer. Optionally a feedback circuit can be inserted into the signal path between the sensor and the amplifier.
Sheltons disclosure fails to teach how the system should really work: In particular just conveying the signal produced by the sensor to the operational amplifier will result in oscillation of the system and distortions generated by the system. The function of the optional feedback circuit is not clearly defined either.
Max Hobelsberger's invention (U.S. appl. No. 07/776,426) functions according to the same principles, a transducer and a pressure sensor are placed inside the housing. Additionally M. Hobelsberger uses the principle of servo control to control the air pressure inside the housing: A controller, together with a closed loop control system, keeps the pressure inside the housing equal to the mean air pressure outside the housing.
However his disclosure does not teach where to arrange the pressure sensor in relation to the rest of the system. This placement of the sensor proves to be crucial to the functioning of the system: A wrong placement of the sensor will cause oscillations and distortions.